Polycarbonate is a well known thermoplastic polymer useful in a variety of settings including electrical components, parts wherein high thermal resistance is important and parts wherein high impact resistance is also significant. This polymer is made by a variety of well known methods including, for example, a lime process, pyridine process, solution process, melt polymerization, and interfacial polymerization. As typical of most reactions the reaction which produces high molecular weight aromatic polycarbonate is not totally selective. There are numerous side reactions also occurring at the same time. Therefore the yield of aromatic polycarbonate is not totally 100%. The side reactions and side products will differ according to the specific process employed including reactants and process conditions. It has recently been discovered in the laboratory of the assignee of this patent application, unknown to that date, that accompanying the production of high molecular weight, linear aromatic polycarbonate is a significant production of cyclic oligomeric dihydric phenol carbonates. Usually the linear oligomers are a reasonable side product in a reaction producing high polymer since it is the oligomer which has not fully reacted and entered the chain. In this case the oligomers not only do not react to enter the chain but also, surprisingly, react intramolecularly and form a cyclic system. Within the labs of the assignee utilizing the interfacial method of preparation, up to about 1.0 weight percent or more of the final product is cyclic oligomeric dihydric phenol carbonate. The cyclic oligomeric dihydric phenol carbonate hereinafter referred to as "cyclic oligomer" is separated from the linear high molecular weight aromatic polycarbonate while in the halogenated solvent methylene chloride. It has been found in the laboratory of the assignee that the addition of a lower ketone, for example acetone, brings about a differential solubility between the cyclic oligomer and the aromatic polycarbonate. The aromatic carbonate is insoluble in a composition containing a sufficient amount of acetone while the cyclic oligomers remain soluble in the solution. Therefore a simple phase separation occurs wherein a substantial proportion of the cyclic oligomers is efficiently separated from the aromatic polycarbonate.
The cyclic oligomers are recovered upon evaporation of the methylene chloride in acetone solution using steam precipitation. However, in order to make the process reasonably economical, both the acetone and methylene chloride must be recovered. In doing so there are certain disadvantages. Two distillation steps are necessary for solvent recovery--the separation of methylene chloride from acetone and also from water. The water is present from the steam precipitation recovery of the polycarbonate. In recovering the acetone from the cyclic oligomer the boiling of the acetone requires energy which is costly. Additionally it is rather difficult to recover methylene chloride, boiling point 40.degree. C., from acetone, boiling point 58.degree. C., and water both quantitatively and in pure form in a relatively inexpensive manner. Finally, a safety issue is present because of the flammability of acetone. Consequently, the prior process utilized to separate cyclic oligomers from aromatic polycarbonate has certain disadvantages.
A new process for separating cyclic oligomers from aromatic polycarbonate has been discovered. It does not utilize further organic solvents. Thus the problems associated with solvent recovery and safety procedures are no longer encountered.